In-situ soil nitrate ion concentration sensor

ABSTRACT

A method and apparatus for near real-time in-situ soil solution measurements is presented. An outer sleeve is placed in soil where ionic concentrations of organic or inorganic species are to be measured. A porous section connects with the outer sleeve (the porous section initially loaded with distilled water) equilibrates with the solution present in soil pores to form a solution to be measured. The initial distilled water is displaced within the porous section by a removable plunger. After substantial equilibration of the solution to be measured within the apparatus, the plunger is removed and a removable probe replaced. The probe may be an Ion Selective Electrode, or a transflection dip probe. The probe then may be used under computer control for measurement of solution properties. The Ion Selective Electrode may measure nitrate (NO 3   − ) concentrations. The transflection dip probe may be read with spectrometer with an input deuterium light source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority from U.S. provisionalapplication Ser. No. 60/986,663, filed on Nov. 9, 2007, which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No. 0410055awarded by the National Science Foundation. The Government has certainrights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to ion concentration measurement, moreparticularly in-situ soil solution ion measurement, and still moreparticularly to the real time in-situ measurement of soil nitrate ionconcentrations.

2. Description of Related Art

Traditional soil analysis consists of digging up or coring a soil sampleof interest, transporting the sample to a laboratory, and then, finally,reading laboratory results. These methodologies are not conducive toreal-time or in-situ measurements.

BRIEF SUMMARY OF THE INVENTION

An aspect of the invention is an in-situ probe that comprises: a tubewith a distal end; and means for measuring one or more chemical speciesthat enters into a distal end of the tube. The distal end of the tubemay contact soil at a porous section of the distal end of the tube. Thesoil is intact ground soil that is to be measured in-situ. Within theporous section may comprise a solution. The solution is, after asufficient time for equilibration, a fluid that has the same propertiesas the fluid present in the interstitial voids between soil particles.Typically such fluid is water with various dissolved ions, and othersmall particulates in suspension.

The means for measuring may comprise measuring a concentration of an ionin the solution that diffuse or flow into the distal end of the tubethrough the porous section. The means for measuring may also comprise:an Ion Selective Electrode, or a nitrate Ion Selective Electrode, or anoptical probe. Still other probes may be used to determine particulatesuspension within the solution. The optical probe may be an Ultraviolet(UV) fiber optic probe.

The optical probe may comprise a transflection dip probe, or otherabsorption-based probe capable of transmitting an input light sourcethrough the sample solution to form an output signal.

Another aspect of the invention is a method of in-situ measurement ofsolution ion measurement in soil that comprises: a. placing ameasurement system, comprising an outer tube sleeve, into a soil that isto be measured, wherein the distal section of the outer tube sleevecomprises a porous section; b. initially contacting a quantity ofdistilled water with the porous section; c. allowing the quantity ofdistilled water to equilibrate with the soil to be measured, through theporous section, so as to equilibrate into a solution to be measured; d.contacting the solution with a probe; and then e. measuring the solutionwith the probe.

The method above may additionally comprise: a. inserting a plunger intothe outer tube sleeve, thereby contacting the distilled water with theporous section; and after the solution equilibrates, b. replacing theplunger with the probe. The probe may be comprised of one or more of agroup of probes consisting of: an Ion Selective Electrode probe, and atransflection dip probe. Furthermore, the Ion Selective Electrode probemay be a nitrate Ion Selective Electrode probe.

The transflection dip probe discussed above may be capable of measuringabsorbance in the range of 235-240 nm, or even 200-1100 nm. Furthermore,the transflection dip probe may specifically measure absorbance at oneor more of the wavelengths selected from the group of wavelengthsconsisting of: 235, 238, and 240 nm.

A still further aspect of the invention is an in-situ ion measurementsystem, that comprises: a. an outer sleeve tube; b. a porous sectionattached to the outer sleeve tube, wherein the porous section allowsdiffusion and bulk fluid flow therethrough; c. a probe placed within theouter sleeve tube, wherein a sensitive portion of the probe contacts ameasurement solution that has flowed or diffused through, orequilibrated through, the porous section.

The probe may be an Ion Selective Probe or a nitrate Ion SelectiveProbe. Other ions may be measured, such as macronutrient ions of:nitrogen, phosphorus, and potassium. Secondary nutrient ions may bemeasured, such as ions of: calcium, sulfur, and magnesium. Further,micronutrient ions may be measured, such as ions of: boron, chlorine,manganese, iron, zinc, copper, molybdenum, and selenium. Common soilcomponents may also be measured for ions of: calcium, magnesium,potassium, and sodium. Other ions inimical to human and animal life maybe measured, such as: arsenic, cadmium, and uranium.

Furthermore, the probe may be a transflection dip probe. Thetransflection dip probe allows absorption of an input light source topass through the measurement solution to form an output signal. Theinput light source may be created by a deuterium light source thatprovides an input light source. Any other light source that illuminatesoptical absorption spectra of a target ion may also be used. An opticalspectrometer may be used to measure an output signal from thetransmission of the input light source through the solution.

By suitable design choices, it may be possible to construct a probe thatsimultaneously measures a plurality of ion concentrations, either with aplurality of Ion Selective Electrodes, or with one or more Ion SelectiveElectrode and one or more transflection dip probes. Additionally, bysuitable design, the transflection dip probe may have a sufficientlybroad measurement range to allow concentration measurements of aplurality of concentrations.

A transflection dip probe may be used for in-situ monitoring ofgroundwater or vadose zone plumes. Within such aqueous sources,contaminants such as benzene, toluene, ethyl benzene, xylenes, styrenes(the foregoing collectively referred to as BTEXS), and or other organicsolvents (in either aqueous or non-aqueous phases) may be measured.

In still other applications, in-situ may be used for the monitoring ofalcoholic fermentation for wine and beer making by sampling alcohol,sugar, or both concentrations.

A computer may be used to control a collection of in-situ measurementsthrough the spectrometer. These spectrometer measurements may be ionconcentration measurements, or more specifically they may be nitrate ionconcentration measurements. The process of sequentially collecting aseries of measurements using the in-situ apparatus may be stored as aprogram executable on a computer readable medium.

The in-situ spectrometer measurements may be measured on one or morecomponents of BTEXS. BTEXS may comprise one or more of a contaminantselected from a group of contaminants consisting of: benzene, toluene,ethyl benzene, xylene, and styrene.

The in-situ spectrometer measurements may comprise one or more of acontaminant selected from a group of contaminants consisting of: apolychlorinated biphenyl, tetrachloroethylene, sulfur dioxide, arsenic,selenium, petroleum, petroleum distillates, petroleum byproducts,hydrocarbon, pentachlorophenol, creosote, a pesticide, a polycyclicaromatic hydrocarbon (PAH), a volatile organic carbon (VOC), a dioxin, adibenzofuran, copper, lead, zinc, hexavalent chromium, cadmium, andmercury.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1A is a side view of an outer sleeve assembly.

FIG. 1B is an exploded assembly view of the outer sleeve and plungerassembly of FIG. 1A.

FIG. 1C is a section C—C taken through FIG. 1A, showing the innercomponents of the overall unit with plunger as assembled.

FIG. 1D is a sectioned side view of an Ion Selective Electrode.

FIG. 1E is a sectioned side view of a transflection dip probe.

FIG. 1F is an exploded assembly view of the outer sleeve assembly ofFIG. 1E with an Ion Selective Electrode.

FIG. 1G is a cross-section taken through FIG. 1F, showing the innercomponents of the overall unit fully assembled with an Ion SelectiveElectrode.

FIG. 2A is a cross-sectional view of a second generation in-situ samplemeasurement assembly, with distilled water occupying a lowest innerportion.

FIG. 2B is a cross-sectional view of a second generation in-situ samplemeasurement assembly, with an improved second generation plungeroccupying nearly all of the interior volume of the sample assembly anddisplacing the distilled water of FIG. 2A up to a porous section.

FIG. 2C is a cross-sectional view of a second generation in-situ samplemeasurement assembly, with an improved Ion Selective Electrode occupyingnearly all of the interior volume of the sample assembly.

FIG. 2D is a cross-sectional view of a second generation in-situ samplemeasurement assembly, with a transflection dip probe occupying nearlyall of the interior volume of the sample assembly, and sampling the testsolution at the lower end of the sample assembly.

FIG. 2E is a cross-sectional view of a second generation in-situ samplemeasurement assembly, with a transflection dip probe occupying nearlyall of the interior volume of the sample assembly, and sampling the testsolution further from the lower end of the sample assembly due to adisplacement attachment.

FIG. 3 is calibration plot of ion selective electrode for a standardnitrate aqueous concentration in parts per million (ppm) versuselectrode potentials in mV taken over six dates within nearly a month.

FIG. 4A is a schematic for an optical setup for transflection dip probemeasurement of ions in solution.

FIG. 4B is a blow up schematic of a sensitive portion of an opticaltransflection dip probe measurement of ions in solution.

FIG. 5 is a plot of nitrate absorbance between about 250 and 350 nm fordifferent nitrate concentrations.

FIG. 6 is a plot of nitrate absorbance versus wavelength in nm forconcentrations for 1, 10, 25, 50, and 100 ppm of nitrate.

FIG. 7 is a plot of nitrate concentration in ppm versus absorbance atwavelengths of 240, 238, and 235 nm.

FIG. 8 is a time-varying plot of absorbance versus wavelength in nm froma saturated diffusion experiment over time periods of 0, 7, 22, 46, and72 hours.

FIG. 9 is a plot of nitrate concentration (ppm) versus time in hours ofthe inside concentration C_(i) due to diffusion, with data points forboth Ion Selective Electrode and optical transflection dip probewavelengths of 235, 238, and 240 nm.

FIG. 10 is a plot of nitrate concentration (ppm in water) versus time inhours of the inside concentration C_(i) due to diffusion with twodifferent initial soil saturation values, 0.434 m³ m⁻³ and 0.347 m³ m⁻³.

FIG. 11 is a natural logarithmic plot of

$\left( {1 - \frac{C_{i}}{C_{ave}}} \right)$versus time in hours, where C_(i) of the inside concentration due todiffusion, and C_(ave) is the average concentration.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inFIG. 1A through FIG. 11. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts, and that themethod may vary as to the specific steps and sequence, without departingfrom the basic concepts as disclosed herein.

DEFINITIONS

The following terms are used herein and are thus defined to assist inunderstanding the description of the invention(s). Those having skill inthe art will understand that these terms are not immutably defined andthat the terms should be interpreted using not only the followingdefinitions but variations thereof as appropriate within the context ofthe invention(s).

“Absorbance” (A), in this invention and as used in spectroscopy, isdefined as

A_(λ) = −log₁₀(I/I_(o))where A_(λ) is the absorbance at a specified wavelength λ, I is theintensity of light that has passed through a sample (transmitted lightintensity) and I_(o) is the original intensity of the light before itenters the sample (the incident light intensity). Absorbancemeasurements are often carried out in analytical chemistry, since theabsorbance of a sample is proportional to the thickness of the sampleand the concentration of the absorbing species in the sample, incontrast to the transmittance

I/I_(o)of a solution, which varies logarithmically with thickness andconcentration.

“Computer” means any device capable of performing the steps, methods, orproducing signals as described herein, including but not limited to: amicroprocessor, a microcontroller, a video processor, a digital statemachine, a field programmable gate array (FGPA), a digital signalprocessor, a collocated integrated memory system with microprocessor andanalog or digital output device, a distributed memory system withmicroprocessor and analog or digital output device connected by digitalor analog signal protocols.

“Computer readable medium” means any source of organized informationthat may be processed by a computer to perform the steps describedherein to result in, store, perform logical operations upon, ortransmit, a flow or a signal flow, including but not limited to: randomaccess memory (RAM), read only memory (ROM), a magnetically readablestorage system; optically readable storage media such as punch cards orprinted matter readable by direct methods or methods of opticalcharacter recognition; other optical storage media such as a compactdisc (CD), a digital versatile disc (DVD), a rewritable CD and/or DVD;electrically readable media such as programmable read only memories(PROMs), electrically erasable programmable read only memories(EEPROMs), field programmable gate arrays (FGPAs), flash random accessmemory (flash RAM); and information transmitted by electromagnetic oroptical methods including, but not limited to, wireless transmission,copper wires, and optical fibers.

“Ion Selective Electrode” (ISE) (also known as a specific ion electrode(SIE)) means a transducer that converts the activity of a specific iondissolved in a solution into an electrical potential that may bemeasured by a voltmeter or pH meter. The voltage is theoreticallydependent on the logarithm of the ionic activity, according to theNernst equation. The sensing part of the electrode is usuallyconstructed as an ion-specific membrane, along with a referenceelectrode. Ion Selective Electrodes are used in biochemical andbiophysical research, where measurements of ionic concentration in anaqueous solution are required, usually on a real time basis.

“Solution” generally, but is not limited to, water and any otherdissolved or suspended material found in a liquid that may be removedfrom soil. In one non-limiting example, nitrate ions are present in suchsolution, and may be measured to determine the amount of nitrateavailable to plants in a specific location of soil.

“Tensiometer” means a device used to determine soil matric potentialψ_(m) (soil water tension) in the vadose zone. The tensiometer consistsof a Plexiglas or plastic tube with a porous ceramic cup, and is filledwith water. The top of the tube has either a built-in vacuum gauge or arubber septum used with a portable handheld reading instrument, whichuses a hypodermic needle to measure the pressure inside the tensiometer.The tensiometer is buried in the soil, and a pump is used to pull apartial vacuum. As water is pulled out of the soil by plants andevaporation, the vacuum inside the tube increases. As water is added tothe soil, the vacuum inside the tube pulls moisture from the soil anddecreases. The actual gage reading will vary according to the type ofsoil, the moisture content, and due to hysteresis, according to thesaturation history of the soil.

Tensiometers are used in irrigation scheduling to help farmers and otherirrigation managers to determine when to water. In conjunction with awater retention curve, tensiometers can be used to determine how much towater. With practice, a tensiometer can be a useful tool for thesepurposes. Tensiometers can also be used in the scientific study of soilsand plants.

A simpler, but somewhat less precise, description of the tensiometer isa device commonly used to measure water pressure in soil. Onetensiometer may be used with other tensiometers to determine thedirection of water flow in a soil profile through determination of soilhydraulic gradient.

“Transflection dip probe” means a device that measures transmission,dispersion, and reflection losses of a solution that it has been dippedor otherwise immersed into. By using such transmission or reflectionover a known distance, an absolute concentration of a specificabsorptive species may be determined with a calibrated light source anda spectrometer.

Introduction

Among the soil nutrients for plant growth, nitrogen (N₂) is one of theessential macronutrients for crop production, mostly taken up by plantsin the form of the nitrate (NO₃ ⁻) ion. The absence of in-situinstrumentation limits the ability to monitor concentration levels ofsoil solution nitrate and to evaluate plant nutrient uptake mechanismsfor specific environmental and agricultural management practices.

New in-situ measurement techniques are presented here that may beapplied to actual growing crops, with soil nitrate concentrationmeasurements recorded electronically in real time, as opposed toextracting soil solutions for time consuming remote laboratorymeasurements. Both methods of nitrate measurement described here use astainless steel solution sampler design comprising a pointed hollow tubeor sleeve.

Methods of Nitrate Measurements

One in-situ nitrate measurement technique uses a nitrate (NO₃—) IonSelective Electrode (ISE) to determine in-situ nitrate concentrations insoil. Laboratory tests have confirmed that by using a one-pointcalibration procedure, the nitrate (NO₃ ⁻) Ion Selective Electrode isable to measure in situ nitrate (NO₃ ⁻) ion concentrations in soilsolution.

A second in-situ nitrate measurement technique measures nitrate ionconcentrations by using transflection dip probes, based on Ultravioletto Visual (UV-VIS) wavelength absorption spectroscopy. This nitratesampler has internal optics that enables a beam of light to be directedthrough the solution in the sampler. Internal optics within thetransflection dip probe are coupled to optical fibers that allow lightto be transmitted into and out of the sampler, enabling analysis with aportable fiber optic spectrometer.

This invention describes both of the above innovative measurementapproaches for use for in-situ nitrate measurements. By analogy, manyother soil-borne ion species may be readily measured.

First Generation In-Situ Ion Measurement

Refer now to FIGS. 1A-1F, which together detail the construction of arepresentative in-situ nitrate (NO₃ ⁻) or other Ion Selective Electrodemeasurement system 100. This system comprises three main parts.

First a stainless steel porous section 102 and stainless steel tube 104are joined by welding 106 or otherwise joining of a transition piece tomatch diameters. The top 108 of the stainless steel tube 104 is threadedto ensuring a pressure tight fitting when inserting two other partsduring the measurement process.

Second, a hollow white Delrin® (DuPont's brand name forpolyoxymethylene) cylinder 110 is used as a volume limiter (also knownas the plunger 110). The sheer volume of the hollow cylinder 110 servesto limit the amount of solution 112 allowed in the solution chamber(formed by the cylinder 110 and the porous section 102). At the centerof the plunger 110, a through hole 114 is drilled through the centerlength to allow excess solution from the porous section 102 to escapeupward into the central body of the plunger 110, thus allowing theplunger 110 to be used as a tensiometer in connection with externalpressure measurement equipment (not shown).

In this manner, the measurement system 100 may be supplied with an extrasource of solution until equilibrium is established with surroundingsoil conditions. An O-ring seal 116 is provided to seal the plunger 110to the inner diameter of the stainless steel tube 104, thus restrainingfluid flow from one end of the stainless steel tube 104 to the other topass only through the central hole 114 of the hollow plunger 110. Aslong as the porous section 102 is completely buried in soil, the overallsystems of FIGS. 1A, 1B and 1C are capable of holding a gage vacuum of700 mbar.

Third, a probe section may comprise either an Ion Selective Electrode(ISE), or an optical probe, either suspended by an inner sleeve tube118. Referring specifically now to FIGS. 1D and 1E, it is found that theinner sleeve tube 118 may serve to support the Ion Specific Electrode inFIG. 1D or optical probe in FIG. 1E inside in a sealed, waterproofattachment on the interconnect end of the probe. The distal end 120 ofthe inner sleeve tube 118 is capable of holding different types of IonSpecific Electrodes 124 when different ion species are to be measured inthe solution rather than the nitrate ion. Further, a multiplicity ofoptical probes 126 may be interchanged as well. These probes have incommon exposure to any fluid in the porous section 102, thus allowingoperation of the sensor portion of the probe.

The inner sleeve tube 118 connects to the distal end 120 through asealed threaded connection 122 that is waterproof. If it is desired toreplace a first Ion Specific Electrode 124 with another Ion SpecificProbe, or an optical probe 126, then the inner sleeve tube 118 sleeve isremoved the outer sleeve assembly 124 and replaced with a new probe.

To measure ion concentrations in the soil solution, the outer sleeveassembly 128 depicted in FIG. 1A is first installed in the soil at adesired depth. Then, the porous section 102 of the outer sleeve assembly128 is filled with a known volume of distilled water. After filling, thewhite Delrin® plunger 110 is installed within the outer sleeve assembly128 and sealed with a top cap section 130.

The sealing action of the top cap section 130, along with the O-ringseal 116 creates a water and airtight seal in the overall unit 132 abovethe porous section 102. By varying the overall length of the stainlesssteel tube 104 and other related components, the top cap section 130 mayalways be accessible above ground for ease of access, or indeed evenburied as desired. The distilled water in the porous section 102 thenundergoes an equilibration with the surrounding soil solution throughdiffusion and pressure differential processes across the porous section102 to become an equilibrated solution 112 with the surrounding soil.

Any vacuum developed inside the central hole 114 of the plunger 110 maybe monitored by a suitable vacuum or pressure transducer in fluidconnection with the central hole 114. After a suitable period of timeequilibrating, the ion concentrations within the porous section 102solution 112 reflect the ion concentrations present in solution in thepore space of the surrounding soil.

When it is initially desired to measure ion concentrations in the poroussection 102, the plunger 110 is removed, and the probe with inner sleeveand probe assembly 134 is inserted through the remaining stainless steeltube 104, and into the porous section 102. Thus, any probe thusinstalled will be sensing the solution 112 in the porous section 102,which is representative of the surrounding soil solution conditions.Thereafter, the optical probe with inner sleeve and probe assembly 134may be left in place to provide continuous in-situ ion measurements asrequired.

However, during the measurement of nitrate concentration with an IonSelective Electrode, the solution 112 of the porous section 102 assemblymay be discharged losing the existing equilibrium pressure that existsbetween the porous section 102 and the surrounding soil. Such a loss ofequilibrium pressure may cause a decrease in the solution 112 inside theporous section 102 assembly and may eventually lead to an inability tomake any meaningful measurement by Ion Selective electrode. To overcomethis potential internal pressure loss problem, a second generation ofthe probe was developed to prevent loss of solution in the poroussection 102 assembly during measurement with either Ion SelectiveElectrodes or optical probes.

Referring now to the overall designs of FIGS. 1A-1G, it may be seen thata conical section 136 is useful for inserting the overall measurementsystem 100 into the ground with minimal disturbance of the surroundingsoil. Additionally, a valve 138 at the top of the measurement system 100allows for the introduction of additional distilled water, oralternatively attachment of vacuum or pressure transducers so thatoperation as a tensiometer of the overall measurement system 100 may beobtained. In place of a valve 138, a pressure fit gland 140 (typicallycomprising one or more deformable elements, not shown), may be used toseal sample tubes 138 or wires 138 exiting the measurement system 100.Alternatively, a rubber septum (not shown) may more simply replace thetop cap assembly 130.

Second Generation In-Situ Sampler

Refer now to FIGS. 2A through 2E, which detail the construction andoperation of the second generation measurement systems 200. The bottomassembly 202 was redesigned to comprise two parts: reservoir cup 204,and porous section 206 (FIGS. 2A and 2B). For measurement operation,about 4 ml of de-ionized or distilled (preferably distilled) water 208is initially placed into the bottom of reservoir cup 204 (FIG. 2A).

A second generation plunger 210 (not hatched so that the other detailsmay be viewed) is inserted into the bottom assembly 202 through thestainless steel tube 212 that displaces the solution 208 initially inthe bottom of the reservoir cup 204, up to the porous section 206 level(FIG. 2B). The solution 214 then in the porous section 206 willequilibrate with surrounding soil solution through diffusion and otherbulk transport processes that may be present. (Assembly diagrams are notshown here, as the assembly process is very similar to those describedin FIGS. 1A-1F, and would merely be repetitive.)

When equilibrium is established between the solution 214 at the poroussection 206 level, the second generation plunger 210 is removed frominside the stainless steel tube 212 and the solution 214 eventuallyresumes its original position 208 in the reservoir cup 204 of FIG. 2A.However, at this point the solution 214 at its initial position 208 isnow an ion-bearing solution, representative of the surrounding soilmatrix within which it had previously been in contact with.

The ion-bearing solution 208 is then measured by inserting either an IonSelective Electrode 216 (FIG. 2C) or an optical probe (218 in FIG. 2D or220 in FIG. 2E). The solution sampler thus is designed in such a waythat the ion-bearing solution 208 remains in the reservoir cup 204during ion measurements with either the Ion Selective Electrode (FIG.2C) or the optical probe (FIGS. 2D and 2E) measurements. All of thesesolution samplers were designed to measure ion concentrations in thesoil solution 208, however, now different concentrations and types ofions in the soil solution 208 may be measured. Of particular interest,nitrate ion concentrations may now be measured.

It should be noted that the Ion Selective Electrode 216 of FIG. 2C mayoperate with a reference pellet connected to the reference chemical 222and an air vent 224. Additionally, the optical probe 218 of FIG. 2D maysimply be immersed in the fluid solution 226 to be measured. In anotherembodiment of FIG. 2E, however, a displacement attachment 230 mayprotrude from the bottom of the optical probe 220 so as to raise thesolution level 228 above the level that would have attained with thedesign of FIG. 2D. Correspondingly, the sensing volume 232 of FIG. 2E iscorrespondingly elevated over the prior design of FIG. 2D.

Second Generation In-Situ Ion Selective Electrode Calibration

Ion Selective Electrodes (ISEs) may be used to measure the concentrationof a specific ion (without limitation, nitrate) in aqueous samples. Thecumbersome calibration procedures of the nitrate Ion Selective Electrode(ISE) were sufficiently time consuming that a less complicated and timeconsuming procedure was developed. A one-point reference Ion SelectiveElectrode calibration procedure was therefore developed.

Refer now to FIG. 3, which is a data plot of standard nitrate ionconcentrations versus Ion Selective Electrode potentials was measuredover a period of nearly a month. Eight specific nitrate concentrationswere created as standards, over a concentration range from 1 to 100 partper million (ppm). All of the nitrate concentrations were tested onvarious listed days within the month.

Assuming a stable and constant slope of calibration curves, an averagedslope value was obtained by averaging the slope values from the previouscalibrations of FIG. 3. By using the averaged slope value, b, a singlepoint calibration equation was obtainedC=C _(ref) e ^(b(mV-mB) ^(ref) ⁾  (1)where C is the solution concentration (ppm); C_(ref) is the referencesolution concentration (ppm); mV is the measured electro-potential ofsolution (mV); b is the averaged slope; mV_(ref) is the measuredelectrical potential of a reference solution (mV).

Absorbance Measurements

Refer now to FIGS. 4A and 4B, which are schematics of an overall ionabsorption analysis system 400. FIG. 4B is a blow up of a particularlycomplex region of FIG. 4A. The absorbance measurements (here, nitrate,but suitable modifications could be made for other ions in solution)were made with an ultraviolet-visible (UV-VIS) fiber optic transflectiondip probe 402 manufactured by Ocean Optics, 830 Douglas Ave., Dunedin,Fla. 34698, USA. This probe was used in conjunction with the OceanOptics SD-2000 UV-VIS spectrometer 404 and a Heraeus Fiberlight UV lightsource 406 (Heraeus Noblelight LLC, Duluth, Ga. 30096, USA). Thespectrometer 404 was coupled to a computer 408 for data acquisition andanalysis.

The dip probe 402 was in the form of a stainless steel cylinder 410 thatcontained two optical fibers (illumination source 412 and signal 414)and a lens 416. UV light from the deuterium lamp was directed into theillumination source fiber 412 of the dip probe 402. This illuminationsource fiber 412 terminated within the probe 402, and the light thatexited from the source fiber 412 terminus 418 impinged upon aplano-convex lens 416 located at the end of the stainless steel cylinder410. This lens 416 directed the light along a path 420 through theliquid 422 (water and ion, calibrated here as with nitrate ions) andonto a surface mirror 424 that was held rigidly in place a fixeddistance from the lens 416. The light that was reflected from the mirror424 returned 426 to the plano-convex lens 416, which focused the returnlight beam onto the returning signal fiber 414.

The return signal fiber 414 carried light back to the spectrometer 404,allowing the liquid absorption spectra of the liquid 422 to be measured.Although not a limitation, the path length used for the present dipprobe configuration was 10 mm.

Nitrate Ion Absorption Characteristics

Refer now to FIG. 5, which is a graph of aqueous nitrate ion absorbancefrom about 250-350 nm. Regardless of concentration, these plots exhibittwo ultraviolet (UV) absorption peaks. A first peak, which is relativelyweak, is centered at about 300 nm. A second, much stronger peak iscentered at about 210 nm. Both peaks are relatively broad such thatuseful absorption data may be obtained at wavelengths away from thepeaks. The optical system used in these experiments could measure UV-VISabsorption spectra from about 200 nm to 700 nm, and report results frommeasurements that employed data from the stronger, second UV absorbancepeak of nitrate at about 210 nm.

However, it was found that for nitrate loadings as high as 100 ppm, theshort-wavelength absorbance values were too high for accuratelymeasuring concentrations for wavelengths less than about 230 nm.Therefore, absorbance spectra at wavelengths in the range 235 nm to 240nm were used instead. This modified absorbance range provided goodresults while still allowing strong (but not completely saturated)signals.

Refer now to FIG. 6, which graphs nitrate absorbance spectra over the235-240 nm range for concentrations ranging from 1 to 100 ppm ofnitrate. These spectra were obtained by placing the transflection dipprobe into nitrate-distilled-water solutions that had been previouslyprepared. The reference spectrum in each case was deionized water.

Refer now to FIG. 7, which shows plots and quadratic curve fits ofcalibration data relating absorbance at a given wavelength to thenitrate ion concentration present. FIG. 7 shows calibration data for thewavelengths of 235 nm, 238 nm and 240 nm, with a high degree ofcorrelation (respectively, R²=0.9999, 0.9987, and 0.9969).

Experimental Results—Diffusion Experiments

To test the performance of the in-situ solution sampler, diffusionexperiments were conducted at two different water content levels ofpacked Oso Flaco fine sand (taken from the Oso Flaco sand dunes nearGuadalupe, Calif., USA). The first water content had a saturation of0.434 m³ m⁻³. The second water content was 37 cm suction equivalentwater content of 0.347 m³ m⁻³.

To start the experiment with fully saturated conditions, a Büchnerfunnel was filled with 100 ppm NO₃— solution. Oso Flaco fine sand waspacked around a supported in-situ solution sampler previously described.The resulting bulk density was 1.50 Mg m⁻³. The diffusion experiment atsaturation was started by adding 12 ml deionized water into the in-situsolution sampler. The changes in concentration with time were measuredwith both Ion Selective Electrode and UV-VIS fiber optic transflectiondip probes during NO₃ ⁻ diffusion from the Oso Flaco soil solution intothe sampler solution of both probes.

Refer now to FIG. 8, which graphs the results of the time-varyingabsorption spectra from the saturated diffusion experiment previouslydescribed. FIG. 8 shows absorbance spectra at different times, such as0, 7, 22, 46, and 72 hours.

Refer to associated FIG. 9, which restates the time-varying nitrate ppmlevels measured at wavelengths of 235 nm, 238 nm and 240 nm of FIG. 8.The differences between the data values for these wavelengths are takento represent the uncertainties in the measurements, which are dependenton the experimental apparatus. Also shown for comparison areexperimental data that were obtained using a nitrate Ion SelectiveElectrode. The data sets are very similar, indicating that both of theseindependent measurement probes work well, and that the optical probeworks equally well at all of the 235 nm, 238 nm and 240 nm wavelengths.

It is also evident from the data of FIG. 9 that the asymptotic value of100 ppm (the initial bulk surrounding nitrate concentration) would takewell in excess of 100 hours to even closely approach. Given the datafrom FIG. 9, the time constant of the diffusion process (as there is nobulk flow in this experimental setup) could readily be calculated with aleast squares curve fit to derived analytical models.

Refer now to FIG. 10, which is a graph showing the time varying nitrateion concentration inside the solution sampler as a function of time withtwo soil water content levels, one with 0.434 m³ m⁻³, and the other at0.347 m³ m⁻³.

For the experiment with saturated Oso Flaco sand (at 0.434 m³ m⁻³), thenitrate ion concentration measured inside the cup ultimately (beyond thetime scale of the graph) increased exponentially close to the finalvalue of 98.44 ppm, as would have been attained theoretically for ahomogeneously mixed solution of the same solution volumes.

For unsaturated case (at 0.347 m³ m⁻³), the measured concentrations atequal times were smaller than at saturated conditions. This clearlyindicates that the diffusion of nitrate ions through the porous sectionat saturated conditions is much faster than in unsaturated conditions.This can be attributed to shorter diffusional pathways and cross sectionareas in the saturated condition.

Model Results—Predicting the Time-Varying Ion Concentrations

A simplified model to predict the time-varying nitrate concentrationsinside the porous section can be developed by using simple electriccircuit analogies. In this model, average nitrate ion concentrationsinside the porous section are denoted C_(i), and average nitrateconcentrations outside the porous section, i.e., in the outer containeris C_(o). In addition, the liquid volumes inside and outside the poroussection are denoted as V_(i) and V_(o) respectively. For simplicity, thevolume of liquid contained within the porous section material itself isassumed to be negligible. The rate, J, that nitrate ions diffuse fromthe outer volume inside the porous section is given by Equation (2):

$\begin{matrix}{J = \frac{C_{o} - C_{i}}{R_{tot}}} & (2)\end{matrix}$where R_(tot) is the overall resistance to mass transfer. In addition, Jmay be related to the time-rates-of-change of the average concentrationsinside and outside the porous section, as shown in Eq. (3):

$\begin{matrix}{J = {{V_{i}\frac{\mathbb{d}C_{i}}{\mathbb{d}t}} = {{- V_{o}}\frac{\mathbb{d}C_{o}}{\mathbb{d}t}}}} & (3)\end{matrix}$

For development of the model here, mass conservation is required inEquation (4):C _(i) V _(i) +C _(o) V _(o) =C _(ave) V _(tot)  (4)By combining Equations (2)-(4) an ordinary differential equation may beformulated, subject to the initial condition C_(i)(0)=0 (imposed due tothe initial starting condition, where distilled water was placed intothe interior or the porous section C_(i)), which may be solved to resultin

C_(ave) = C_(o)(0) * ((1 + V_(i)/V_(o))⁻¹,where C_(i)(0) and C_(o)(0) are the nitrate concentrations at the timet=0. After rearranging the solution of the ordinary differentialequation, Equation 5 may be obtained

$\begin{matrix}{{\ln\left( {1 - \frac{C_{i}}{C_{ave}}} \right)} = {- {at}}} & (5)\end{matrix}$such that a plot of

$\ln\left( {1 - \frac{C_{i}}{C_{ave}}} \right)$as a function of time should yield a straight line with the slope −a.

Experimental data from a diffusion experiment with the region inside theporous section initially filled with deionized water, i.e., withC_(i)(0)=0, are shown in FIG. (11).

Refer now to FIG. 11, which is a plot of

$\ln\left( {1 - \frac{C_{i}}{C_{ave}}} \right)$versus time. It is apparent, then, that the quantity

$\ln\left( {1 - \frac{C_{i}}{C_{ave}}} \right)$varies linearly with time. By measuring the slope of the data, whichdetermines the variable a, the overall resistance to nitrate diffusion,i.e. R_(tot), may be determined by using the definition of the slope a.This overall resistance may be related to the effective diffusioncoefficient of porous cup to nitrate by assuming that nitrate diffusionthrough the wall is quasi-steady, and solving Laplace's equation incylindrical coordinates

$\begin{matrix}{R_{tot} = \frac{\ln\left( {r_{iw}/r_{i}} \right)}{2\;\pi\;{lD}_{E}}} & (6)\end{matrix}$Where l is inside height of the porous cylindrical section, r_(iw) isthe outside radius, r_(i) is the inside radius of the porous cup, D_(E)is the effective diffusion coefficient of the porous cup.

CONCLUSION

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

1. A method of in-situ measurement of solution ion measurement in soil,comprising: placing a measurement system, comprising an outer tubesleeve, into a soil that is to be measured, wherein the distal sectionof the outer tube sleeve comprises a porous section; initiallycontacting a quantity of distilled water with the porous section;allowing the quantity of distilled water to equilibrate with a soilsolution to be measured, through the porous section, so as toequilibrate into a solution to be measured; contacting the solution tobe measured with a probe; measuring the solution to be measured with theprobe; inserting a plunger into the outer tube sleeve, therebycontacting the distilled water with the porous section; and after thesolution equilibrates; and replacing the plunger with the probe.
 2. Themethod of in-situ measurement of solution ion measurement in soil ofclaim 1, wherein the probe is selected from a group of probes consistingof: an Ion Selective Electrode probe, and a transflection dip probe. 3.The method of in-situ measurement of solution ion measurement in soil ofclaim 2, wherein the Ion Selective Electrode probe is a nitrate IonSelective Electrode probe.
 4. The method of in-situ measurement ofsolution ion measurement in soil of claim 2, wherein the transflectiondip probe is capable of measuring absorbance in the range of 235 nm to240 nm.
 5. The method of in-situ measurement of solution ion measurementin soil of claim 4, wherein the transflection dip probe measuresabsorbance at one or more of the wavelengths selected from the group ofwavelengths consisting of: 235 nm, 238 nm, and 240 nm.